1. Field of the Invention
This invention relates to optical waveform measuring devices, and more particularly to an optical waveform measuring device with a photodetector having a photocathode, such as a streak camera, which is suitable for measuring rays of low intensity light in an infrared ray region at high speed with high sensitivity.
2. Description of the Related Art
There are a variety of means available for measuring the transient behavior of ultra-high speed optical phenomeana. One of the measuring means employs a streak camera in which an incident light beam is converted into an electron beam at the photocathode of the streak tube. The electron beam is allowed to sweep at high speed, so that the intensity of an incident light beam, that changes with time, is measured as a variation in luminance with respect to position on the phosphor screen of the streak tube.
As shown in FIG. 17, an essential component of the streak camera, namely, a streak tube 13 comprises a photocathode 14 for converting light (slit image) into an electron image. The light is applied through a slit plate 10 and an image is formed by a lens 12 in the input optical system. The streak tube 13 also comprises a mesh-type accelerating electrode 16 for accelerating the electron image provided by the photocathode 14; deflecting electrodes 22 for deflecting the electron beam accelerated by the accelerating electrode 16 in a direction (that is a vertical direction in FIG. 17) perpendicular to the longitudinal direction of the slit at high speed; and a phosphor screen 26 for converting the electron image deflected by the deflecting electrodes 22 into an optical image (i.e., a streak image which is a luminance data image in which the vertical axis represents the lapse of time).
Further, as depicted in FIG. 17, the streak tube 13 may also include a focusing electrode for focusing the electron beam accelerated by the accelerating electrodes 16 to a certain degree; an aperture electrode (or anode) 20 for further accelerating the electron beam, a sweep circuit 23 for applying a predetermined sweep voltage across the deflecting electrodes 22 in synchronization with the passage of the electron beam; a micro-channel plate (MCP) 24 provided in front of the phosphor screen 26 to multiply the number of electrons passed through the deflecting electrodes 22; a conical shield electrode 25 provided on the input side of the MCP 24, for blocking the electrons deflected out of the effective sweep region of the phosphor screen to improve the accuracy of measurement; and an image pick-up means 28 comprising a high sensitivity image pick-up device such as an SIT camera or CCD camera for recording the streak image through a lens 27 in the output optical system.
Roughly stated, the above-described streak camera is classified in a single sweep type streak camera and synchro scan type streak camera depending on the operating principle employed; i.e., the sweep system employed. In the single sweep type streak camera, a linear sweep is carried out using an ultra-high speed sawtooth wave up to several kilo-Hertz (KHz) in synchronization with a pulse laser beam. In the synchro scan type streak camera, a high-speed repetitive sweep is carried out with a sine wave of 80 to 160 MHz in synchronization with a laser beam. In addition to the above-described two types of streak cameras, a synchronous blanking type streak camera has been developed in which an elliptic sweep is carried out. As shown in FIG. 18, the return portion of the elliptic sweep is shifted sideways so that the electron beam may not go across the phosphor screen 26.
The above-described conventional streak cameras have been disclosed, for instance, by Japanese patent application publication Nos. 44622/1981, 40709/1982 and 40712/1982, Japanese patent application (OPI) No. 58745/1984 and 183857/1986 (the term "OPI" as used herein means an "unexamined published application"), U.S. Pat. Nos. 4,232,333 4,352,127, 4,611,920, and 4,661,694, and GB Pat. Nos. 2042163, 2044588 and 2131165.
The above-described method using the streak camera is a pure-electronic direct method excellent in time resolution and in detection sensitivity. The method can measure single shot (non-repetitive) phenomena. In addition, since the streak image is originally two-dimensional, the method can be used for two-dimensional measurement such as time-resolved spectroscopic measurement or space-and-time-resolved measurement. Furthermore, when the materials of the photocathode and the incident window are selected, the method can perform measurement over a wide range of spectral sensitivity from near infrared region through vacuum ultraviolet region to X-ray region.
In addition, as shown in FIG. 19, a sampling type optical oscilloscope having a sampling streak tube 30 has been put in practical use. A slit board 32 for spatially limiting a streak image in a streak camera is provided, to electrically sample the streak image.
In FIG. 19, a photo-detector 34 detects the intensity of light that is emitted by a phosphor screen 26 when an electron beam strikes against the latter. Photodetector 34 may be a photo-multiplier tube, a high sensitivity photodiode, an avalanche photodiode, or a PIN photodiode.
The sampling type optical oscilloscope has been disclosed, for instance, by Japanese patent application (OPI) Nos. 104519/1984 and 135330/1984, U.S. Pat. Nos. 4,645,918 and 4,694,154 and GB Pat. No. 2133875.
A drawback of the devices and methods previously described is that they are not sufficiently sensitive and efficient in photo-electric conversion when used for measurement of low intensity light beams. When wavelengths are in the infrared region, for instance with wavelengths of 1.3 .mu.m and 1.5 .mu.m as is extensively employed for optical communication, the photocathode 14 used is an S-1 photocathode. The quantum (conversion) efficiency of the S-1 photocathode is on the order of 10.sup.-4 to 10.sup.-5 for wavelengths of 1.3 .mu.m, and on the order to 10.sup.-6 for wavelengths of 1.6 .mu.m. Thus, efficiency is very low. This is a fatal drawback if, for example, the light is passed through a long distance optical fiber, or is used for measurement in the field of photo-counting optical communication.